My Beloved Brontosaurus

by Brian Switek

  In 2011, the zoologists Jan Werner and Eva-Maria Griebeler reinvestigated the idea proposed by Janis and Carrano. They found that dinosaurs really did have a reproductive edge. By laying eggs and leaving fast-growing offspring early in life, dinosaurs were not energetically taxed in the same way as mammals, and thus had the biological flexibility to be adapted to sizes that may represent the limit of how large terrestrial animals can get. And this reproductive and life-history difference might account for why there were so many huge dinosaurs while there have always been few behemoth-class mammals. Dinosaurs could lay frequent clutches of eggs no matter how big they were, but big mammals might produce only one offspring every few years. This makes large mammals much less prolific and less likely to recover if a population comes under stress. Slow reproduction, which limits their numbers, is a cost of large body size when parental investment is high. Dinosaurs, by contrast, experienced no such cost for being big. High reproductive throughput and low parental investment did not require that they become giants, but the way sauropods kickstarted the next generation made it possible for absolutely enormous dinosaurs to evolve.

  To grow to astonishing sizes, though, hatchling sauropods had to survive. They may not have received much help from their parents. Evidence from dinosaur nest sites and from the behavior of birds and crocodylians hints that dinosaur parents looked after their nests and provided some post-hatching care, as we’ve seen; but baby dinosaurs may have been on their own once they left the nest, and led separate lives from adults of the same species.

  Skull shape is one clue. The skull of a juvenile Diplodocus found at Dinosaur National Monument has a differently shaped snout than the adult dinosaurs. While mature Diplodocus had squared muzzles suited to cropping low-lying plants, the juvenile form had a rounded mouth adapted to browsing; a young sauropod had to be picky about what food it ate, and it plucked specific, nutritious plants to fuel its growth spurt. The juveniles had different needs, diets, and habits than the adults. And, over the past few years, paleontologists have uncovered bonebeds containing only juvenile animals. From Triceratops to the sauropod Alamosaurus, young dinosaurs seemed to stick together in their own cohorts before later joining a breeding group or going solo. Dinosaurs may have had a multistage life history that dictated whom they associated with and when. Glimmerings of their social lives are left in the rock record.

  Six

  Dinosaur Society

  The Cleveland-Lloyd Dinosaur Quarry is easy to miss. Driving seventy miles an hour down the lonely, parched stretch of U.S. Route 6 between Price and Moab, Utah—the first place I ever saw an iconic tumbleweed float lazily across the road—it’s not uncommon for dinosaur lovers like me to blow past the brown sign marking the start of a dirt road that winds past farms and rock exposures to an arid landscape of red, gray, and green badlands. Or maybe it’s my fault for being distracted by the lovely scenery. The highway leading up to the quarry turnoff—part of Utah and Colorado’s scenic “Dinosaur Diamond” byway system—is flanked by barren rock faces. Little domes of sediment flanking the highway give way to large swaths of exposed geologic time that glow when the evening sun strikes them at just the right angle. The panorama doesn’t whisper the truth of Deep Time—it practically screams it. How anyone in this age can believe that all of this geologic grandeur was created in a matter of days is totally beyond me. The strata, deeply set in rainbow colors, highlight the almost incompressible depth of time and could never have been laid down by a mere flood. Ages are stacked upon ages, naked in the baking western sun. Time is evident everywhere.

  The first time I navigated my wimpy little red sedan past the rock formations and over the rough road, I was greeted by a locked set of battered yellow gates just a mile from the quarry entrance. Apparently the site is so remote that the Bureau of Land Management keeps the visitor center open only during weekends, except in the summer season. Too bad I didn’t find that out until I got there, just one day before the official summer hours started. Cursing my stupidity, I started my three-hour drive back to Salt Lake City.

  I tried again the following week, anxious to see the bonebed. This time I had company: my patient wife was along for the ride. Tracey’s not as dinosaur-crazed as I am—botany is her scientific love—but she’ll take any chance to get out and explore Utah’s scenery. Ever since we first visited Utah in 2009 and decided we wanted to move west, every trip we take is split between appreciating the local wilderness and, of course, going out of our way to see whatever dinosaurs might be nearby. It was truly a happy stroke of luck. By moving to Utah, she could immerse herself in unfamiliar ecosystems, I’d get to visit the dinosaur haunts I had yearned to see for so long, and we’d both get a chance to live out our dreams in a beautiful landscape studded with extravagant fossil riches. And while I missed the chance to visit Cleveland-Lloyd my first summer in Utah, I knew I had to take the earliest opportunity I could get during my second year to see one of the most important dinosaur sites ever found.

  Thankfully, on my second attempt I found an easier way to reach the quarry, over paved roads that connect suburban Price to the llama and cattle ranches scattered around the rural town of Elmo, just outside the park. We still had a few miles on the dirt roads, and when we rolled down our windows to let the breeze make up for our broken A/C, fine dust swirled around the car, coating everything. I followed the BLM signposts toward the quarry, constantly checking the clock to see how much time we’d have before the site closed. I didn’t want to waste a second on our way to the dinosaur graveyard.

  * * *

  Strangely, my wife and I were the only visitors to a long, low building of glass and stone, set among the vacant benches of a picnic area. Even though the quarry contains the remains of dozens of dinosaurs, and is the locale where several species of sharp-toothed theropod dinosaurs were first discovered, the site feels like a secret that only attracts people who already know it’s there. The sign out on the highway gives nothing but the name of the quarry, a mundane-sounding title outside of the word “dinosaur.” Maybe more tourists would pull off the interstate if the sign said “Allosaurus Death Trap This Way!”

  The Cleveland-Lloyd Dinosaur Quarry contains one of the Jurassic’s most perplexing mysteries. The star of that mystery stands, jaws agape, in the quarry’s visitor center—and I make a beeline for the dinosaur I have driven three hours (twice!) to see. Just beyond a glass case of jet-black skulls—highlighting some of the charismatic dinosaurs found at the site—a round enclosure fences in a large Allosaurus. I have heard about this reconstruction, and I immediately see that this is a mix-and-match skeleton. While the theropod’s body was reconstructed based on bones found at the quarry, the skull is a cast of a specimen found hours away at Dinosaur National Monument. And, oddly, the Allosaurus reconstruction at Dinosaur National Monument is based on fossils from Cleveland-Lloyd! Together, both dramatic bonebeds outline the life and times of the Jurassic’s most prolific predatory dinosaur.

  Allosaurus has often been cast as the wimpier precursor to Tyrannosaurus. But this isn’t really fair, or accurate. Until the 1990s, paleontologists grouped all large flesh-tearing dinosaurs into a single group—the “Carnosauria.” In this system, the apex predator of Jurassic North America undoubtedly set the stage for the even bigger Cretaceous hunters. But then paleontologists recognized that the Carnosauria was really a hodgepodge of very different dinosaurs that belonged to various hypercarnivorous lineages. Not only was Allosaurus the iconic form of a particular group of giant sauropod killers—called Allosauroids—but rare specimens have hinted that it got to be just as big as Tyrannosaurus. Allosaurus was not the milder precursor of prehistory’s most famous tyrant, but an agile predator of frightening size and aspect. And, based on the Cleveland-Lloyd fossils, some paleontologists have speculated that these apatosaur eaters may have hunted in packs.

  Allosaurus was the most common big predator in Late Jurassic Utah. Paleontologists are still trying to figure out why this 150-million-year-old carnivor
e was so abundant on the ancient floodplains it stalked. (Photograph by the author at the Natural History Museum of Utah)

  Since paleontologists started working Cleveland-Lloyd’s Jurassic graveyard in 1927, the remains of more than forty-six Allosaurus have been extracted from the prehistoric jumble. Most are isolated and scattered in a slurry of osteological remnants; a few crushed bones show that other dinosaurs stepped on the remains of their deceased and defleshed comrades. Paleontologists calculated the minimum Allosaurus count based on the number of left femora found at the site, but not every Allosaurus buried here contributed the same skeletal element to the bonebed. What’s more, only about a third of the bonebed has actually been excavated. There are probably many more Allosaurus here than we’ll ever know.

  Bits of other predators—such as the more massive, knife-toothed carnivore Torvosaurus and a particularly big Ceratosaurus—have turned up in the same deposit, as well as bones from Stegosaurus, the delicately bulky Barosaurus, and the blunt-headed sauropod Camarasaurus. The early tyrannosaur Stokesosaurus and the enigmatic theropod Marshosaurus are here, too, and were first discovered in this quarry. But no dinosaur comes anywhere close to the abundance of Allosaurus, which is far more numerous than all the other dinosaurs put together.

  The yawning Allosaurus in the Cleveland-Lloyd visitor center is a composite skeleton that represents the lives of many who died here. And these battered black fossils formed the standard image of what Allosaurus was like. Utah’s first state paleontologist, James Madsen, used the Cleveland-Lloyd fossils to catalog every last bone in the Allosaurus skeleton in a classic monograph, and Allosaurus skeletons based on this quarry’s bones can be seen in museums all over the world. When I asked the artist Glendon Mellow to design an Allosaurus tattoo for my right arm, he went for inspiration to the Royal Ontario Museum, where an Allosaurus skeleton modeled on the Cleveland-Lloyd fossils has stood for years. If you see an Allosaurus in a museum, there’s a good chance it was based on bones from this isolated spot in Utah.

  The buildings that cover the exposed portions of the actual bonebed are a short walk from the visitor center. Dinosaur National Monument it’s not, but the BLM has left some fossils in place and installed charcoal-colored casts to re-create an image of just how dense the bonebed is. This is a dinosaurian mess. Skull elements, ribs, vertebrae, and limb pieces are strewn across the exposed rock. The Cleveland-Lloyd Quarry is fossil chaos.

  No one knows what created the bonebed, or why Allosaurus unquestionably dominates the assemblage. The sheer number and density of carnivores hint that this was a predator trap, not unlike the much more recent saber-toothed cat– and dire wolf–rich La Brea asphalt seeps in the middle of Los Angeles, California. The classic scenario of the Cleveland-Lloyd’s history, envisioned by paleontologists like Madsen, goes something like this: During the height of a Jurassic drought, a dehydrated Stegosaurus or Camarasaurus discovered one of the few ponds on the baked landscape. As the herbivore dipped its head down to drink, its columnar limbs broke through the cracked surface to the sucking mud beneath, and its last hope for life turned into its ultimate doom. There was no way it could escape, and, when it perished, the reek from the victim’s rotting flesh attracted opportunistic carnivores for miles around—that is, if its agonized cries hadn’t already called in Allosaurus. But when those Allosaurus tucked into the free meal, they suffered the same fate. Much like an abandoned lobster pot, the putrid pond just kept killing, season after season. Since Allosaurus was the most common predator on the landscape, and the overwhelming majority of fossils represent carnivorous dinosaurs, such a scenario would explain the unbalanced collection of bones.

  Not everyone agrees that the quarry was a predator trap. The geologic context is ambiguous. Where some see a mucky pond, others see an accumulation of dinosaurs that died in a drought, or a mass of bones washed in from another location. Everyone who studies the quarry has an opinion about what happened here around 150 million years ago. And even within these scenarios, there are other mysteries. If the site was once a predator trap, then why was Allosaurus so common while other big predators were so rare? Were Allosaurus really that abundant, or did something else skew the sample in their favor?

  One possibility is that the Allosaurus at Cleveland-Lloyd didn’t travel alone. What if the site doesn’t represent an accumulation of lone hunters, but a jumble of families or social groups? The Natural History Museum of Utah—the institution that houses much of the Cleveland-Lloyd collection—brought this idea to life when they opened a new paleontology hall in 2011. Stuck in the Jurassic mud, a hapless skeletal Barosaurus arches its magnificent neck into the air as an Allosaurus family mocks and harasses the giant. One impatient little Allosaurus perches on the sauropod’s back, digging skeletal claws into flesh that isn’t there. The vignette is a brutal Jurassic buffet.

  The Allosaurus assemblage at Cleveland-Lloyd could be a clue that these dinosaurs worked together to bring down big game. But as far as the evidence goes, there’s no way to know for certain. The bonebed was built up over weeks or even years. Dozens of Allosaurus are buried at this one place, but why they are all there is a mystery no one has solved. The dinosaurs were buried together, but this doesn’t necessarily mean that they lived in rapacious packs. In this case, it’s impossible to tell whether the site represents a small number of Allosaurus battalions or whether the graveyard is home to many solitary animals. Cleveland-Lloyd is a Mesozoic cold case.

  The eastern Utah quarry isn’t the only one of its kind. Dinosaur bonebeds found all over the world have raised the possibility that some dinosaurs were social creatures that lived and died together. And if any dinosaur epitomizes the confusing nature of mass graves, it’s Deinonychus. All you have to do is recall Jurassic Park to see why.

  While Tyrannosaurus embodied brute strength—a force of nature that could break through walls and crush barriers underfoot—Velociraptor personified dinosaurian stealth and cunning. However, what we think of as Velociraptor was really Deinonychus. The name change was thanks to a book published five years before Spielberg’s special effects ripped up the celluloid. In his Predatory Dinosaurs of the World, published in 1988, the paleoartist Gregory S. Paul imposed his own unique naming scheme on dinosaurs. He decided to lump Deinonychus—a sickle-clawed predator John Ostrom named in 1969 from partial skeletons discovered in Montana—with Velociraptor, a similar but smaller killer that Henry Fairfield Osborn named in 1924 from bones found in Mongolia. Since Velociraptor had been named first, Paul reclassified the larger and different Deinonychus as Velociraptor. Paleontologists objected to the name change, but, alas, Michael Crichton read Paul’s book while he was researching Jurassic Park, and he renamed the beefier, more formidable Deinonychus in the novel. An actual Velociraptor wouldn’t have been very threatening. While exceptionally well armed, the predator would have been about the size of a turkey, too small to consider a full-grown human a meal.

  What made Crichton’s fictionally enhanced raptors so deadly was their intelligence. Tyrannosaurus was a single-minded killer, but Velociraptor bluffed and feinted to draw victims into their traps. The idea came straight from what the paleontologist John Ostrom hypothesized about the site where Deinonychus was originally found. The quarry, tucked away in central Montana, contained the skeleton of a Tenontosaurus and several partial Deinonychus. The unfortunate herbivore—a member of the ornithischian tribe, cousin to the more familiar Iguanodon—didn’t have any spikes, plates, or other arrangements. The plain dinosaur had a beaked mouth and a long tail, could have walked either on all fours or on two legs, and seemed like the perfect prey for a predator looking for a relatively defenseless meal. Since Ostrom and his colleagues found several partial skeletons of the carnivore Deinonychus—a svelte, muscular predator equipped with retractable toe claws able to sink into the flesh of prey—in the same quarry, it appeared as if a pack of predators had attacked the Tenontosaurus. Some perished, but others won out and enjoyed the greasy spoils of their victory. An abu
ndance of Deinonychus teeth—shed as the dinosaurs dismembered the Tenontosaurus—announced the gruesome victory of the carnivores.

  The ability to cooperate immediately set Deinonychus apart. Previously, paleontologists had thought that predatory dinosaurs were loners that caught prey on their own and selfishly devoured their spoils. The raptors were different. Spielberg’s Velociraptor popularized the military tactics of these dinosaurs. Robert Muldoon’s last words—before he was mauled and presumably eaten by a Velociraptor—were “clever girl,” after all.

  Yet the quarry Ostrom described showed only that partial dinosaurs were buried together. In 2007, the paleontologists Brian Roach and Daniel Brinkman of the Yale Peabody Museum reviewed the available evidence and determined that the case for pack-hunting Deinonychus was not so straightforward. Rather than working together, Roach and Brinkman hypothesized, the dinosaurs competed with each other, and the Deinonychus carcasses at the site were individuals killed in the struggle for the meaty Tenontosaurus. Naturalists have observed similar behavior among modern Komodo dragons—each lizard is working in its own interest, even as multiple animals gravitate toward the same carcass. In short, competition for fleshy morsels may have killed these Deinonychus, not a botched attempt to score a major meal. Much like the Allosaurus bonebed and other predatory dinosaur accumulations, the Deinonychus graveyard can’t be taken as direct evidence that these carnivores acted in coordinated panzer divisions.

  While the Allosaurus and Deinonychus quarries are difficult to interpret, there is at least one bonebed that reflects dinosaur social behavior. This site, tucked away within Canada’s Dinosaur Provincial Park, records a horrific catastrophe that killed dozens of dinosaurs at once. The victims were Centrosaurus—the iconic horned dinosaur that roamed Cretaceous Alberta around 75 million years ago. In general form, they were typical ceratopsids; Centrosaurus had stout bodies with thick limbs for four-footed locomotion. But what distinguished Centrosaurus was the ornamentation on the dinosaur’s skull. Centrosaurus bore a long, slightly curved horn on its nose, lacked brow horns, and had a combination of hooks and hornlets on its frill.